A. Field of the Invention
The present invention relates generally to the communications field, and, more particularly to a fiber optic cable tensioning and positioning apparatus and method for tensioning and positioning a fiber optic cable using the same.
B. Description of the Related Art
Along with the increasing prominence of the Internet has come the wide-ranging demand for increased communications capabilities, including more channels and greater bandwidth per channel. Optical media, such as fiber optic cables, promise an economical alternative to electrical conductors for high-bandwidth long-distance communications. A typical fiber optic cable includes a silica core (glass optical fiber), a silica cladding, and a protective coating. The glass optical fibers of fiber optic cables have very small diameters, which are susceptible to external influences such as mechanical stress and environmental conditions. The index of refraction of the core is higher than the index of refraction of the cladding to promote internal reflection of light propagating down the core.
An optical fiber diffraction grating can output light having a specific reflection wavelength upon reception of incident light. Owing to this advantage, a great deal of attention has recently been paid to the optical fiber diffraction grating as an important optical part in a wavelength division multiplex (WDM) optical transmission communication system which multiplexes and transmits optical signals having different wavelengths through one optical fiber.
The signal carrying ability of fiber optic cables is due in part to the capability of producing long longitudinally-uniform optical fibers. However, longitudinal variations in index of refraction, e.g., those associated with refractive-index gratings, can be included in the fiber optic cables to affect throughgoing pulses in useful ways. Gratings can be grouped into short-period, e.g., about 0.5 micron (xcexcm), or long-period, e.g., about 200xcexcm, gratings. Short-period gratings can reflect incident light of a particular wavelength back on itself in the fiber. Long-period gratings can couple incident light of a particular wavelength into other co-propagating modes on the fiber. Some of these other co-propagating modes may be lost, so the overall effect of the long-period grating can be to selectively block certain wavelengths from propagating efficiently through the fiber.
While there are many methods for establishing a diffraction (or refractive-index) grating within a fiber, one method involves exposing photosensitive glass optical fibers to patterned light, via lasers. The index of refraction of certain fiber-optic materials, such as germanium-doped silica, is changed upon exposure to mid-ultra-violet (mid-UV) light, e.g., wavelengths between 190 nanometers (nm) and 270 nm. The lasers are used to etch lines in the glass optical fiber that is exposed (the coating removed) in the fiber optic cable.
In order to precisely form a refractive-index grating within a fiber, it is preferable to apply a repeatable, uniform tension on the fiber optic cable. A uniform tension ensures that the grating period is consistent across the grating length. A repeatable tension ensures grating period consistency from fiber to fiber. If different tensions are applied from one fiber to the next, the fibers will relax by different amounts and thereby cause different spacings between grating lines. In other words, the fiber is somewhat elastic and will stretch when tension is applied and relax when the tension is released. Thus, applying inconsistent amounts of tension to a series of fibers being etched will result in an inconsistent grating period. The grating period tolerance for optical communications equipment is extremely demanding and will not admit such inconsistencies.
Another preferable feature would be to have an apparatus that is able to precisely position the fiber in a repeatable manner. Otherwise, the grating laser beam(s) will need to be aligned for each etching which slows down the manufacturing process and is quite inefficient.
Tensioning the fiber may also help reduce grating period inconsistencies in another way. More specifically, if the fiber is allowed to sag between two points it will form a catenary curve. Projecting a planar grating pattern on a catenary curve may result in a change in grating period across the grating length. Tensioning the fiber reduces or even eliminates the curvature of the catenary and, thereby, improves the grating period consistency. A repeatable tension force further improves grating period consistency from one fiber etching to the next.
Conventional fiber tensioning apparatuses must be finessed a technician to tension the fiber optic cable. Thus, these apparatuses suffer from the potential for human error and fail to provide a repeatable, uniform tension in the fiber optic cable while etch lines are formed in the glass optical fiber. Even if a skilled technician accurately tensions a particular fiber optic cable, it is virtually impossible for the technician to provide the same tension for a series of cables.
Thus, there is a need in the art to provide an apparatus and a method for accurately and consistently tensioning a fiber optic cable, as well as uniformly tensioning a series of fiber optic cables that are to have identical refractive-index gratings.
The present invention solves the problems of the related art by providing an apparatus and method for uniformly and consistently tensioning and positioning a fiber optic cable that eliminates the potential for human error by a technician. The apparatus and method of the present invention relies upon gravity to provide a uniform, repeatable tension to a fiber optic cable, as will be described more fully below. The apparatus and method is thus useful for uniformly tensioning a multitude of fiber optic cables that are to have identical refractive-index gratings.
In accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method for tensioning and positioning a fiber optic cable, including: securing a first portion of the fiber optic cable to a first support; securing a second portion of the fiber optic cable to a second support; and creating a gravity-assisted moment arm with the second support to uniformly and repeatably tension and position the fiber optic cable between the first and second supports.
Further in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method for forming a refractive-index grating in a fiber optic cable, including: securing a first portion of the fiber optic cable to a first support; securing a second portion of the fiber optic cable to a second support; creating a gravity-assisted moment arm with the second support to uniformly and repeatably tension and position the fiber optic cable between the first and second supports; and etching grating lines in the fiber optic cable after the fiber optic cable has been uniformly and repeatably tensioned and positioned.
Still further in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method for calibrating a fiber optic cable tensioning and positioning apparatus having a first support and a second support rotatable relative to the first support, including: securing a first portion of the fiber optic cable to the first support; securing a second portion of the fiber optic cable to the second support; measuring a diffraction grating provided in the untensioned fiber optic cable; creating a gravity-assisted moment arm with the second support to uniformly tension the fiber optic cable between the first and second supports; measuring the diffraction grating provided in the uniformly tensioned fiber optic cable; and comparing the measured diffraction grating of the untensioned fiber optic cable to the measured diffraction grating of the tensioned fiber optic cable to calculate the tension applied to the fiber optic cable.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.